The Island of Terror by H.C. McNeile

When intrepid adventurer, Jim Maitland, returns to England for a brief visit, he meets a charming young woman named Judy Draycott, who solicits his help in a perilous matter. She relates the story of her brother, Arthur—drifting in South America until he meets an old sailor who, on his deathbed, tells him about a hoard of buried treasure. When Arthur is shot during an attempt to return to London, Maitland persuades his cousin, Percy, to accompany him to Lone Tree Island, where the treasure is allegedly buried. But what can they do with only half a map? And can they evade the undesirables on their trail?